There is known an air-operated percussive action machine (cf., USSR Inventor's Certificate No. 238,424, IPC E 02f 5/18) comprising a cylindrical housing, a hammer having a port and a cavity in its tail portion, a threaded air distribution tube, and a nut provided with passages.
The air distribution tube occupying the interior of the hammer is of stepped configuration. End face edges of the larger step act to control air distribution, i.e., time the inlet of compressed air to the return stroke chamber of the hammer and discharge thereof to the outside. The step of smaller diameter has a threaded portion to connect the air distribution tube to the housing of the machine. By unscrewing the tube from the housing the end face edges of the larger diameter step are displaced to ensure an advance inlet of compressed air to the return stroke chamber, whereby inpacts of the hammer on the front portion of the housing are prevented. Conversely, a delayed exhaust of compressed air causes a longer stroke of the hammer, whereby the impacts are delivered on the rear portion of the housing for the machine to operate in the reverse percussive action mode.
In order to change the percussive action of the machine, it is necessary to stop the delivery of compressed air and screw the tube into the housing. The end face edges of the larger diameter step are therefore moved forward to cause a delayed (as compared with the reverse percussive action) inlet of compressed air to the front working chamber and advanced exhaust thereof to the outside. This determines operation of the machine in the forward percussive action mode, when the impacts are delivered exclusively on the front end of the housing.
The abovedescribed arrangement of the percussive action machine suffers from the following disadvantages:
(a) much time required for switching the machine over from one percussive action mode to the other, since such a procedure involves terminating the supply of compressed air, disconnecting the air-feeding hoses, and turning the tube 10 to 14 revolutions, or repeating these procedures in the reverse successive when changing the percussive action of the machine from the forward to the return travel in the hole;
(b) affected reliability in operation due to that one of the elements of the machine structure, particularly the tube, extends away from the housing, whereby dirt is liabile to plug the threads and jam the machine; and
(c) excessive amount of manual labour for changing the percussive action mode of the machine.
There is also known an air-operated reversible percussive action machine (cf., West German Pat. No. 2,340,751; IPC E 21b 7/00) comprising a housing, a hammer having a port and a cavity in its tail portion to reciprocate inside the housing, a tube having projections and recesses provided at the step of smaller diameter and secured at a flange provided with a hole and grooves, and a lock means with a cable. For reversing the percussive action of the machine it is sufficient to pull the cable, release the lock means, and then to turn the tube so that its projections would align with the grooves of the flange, whereby under the action of compressed air the tube enters the grooves by moving axially.
However, the above arrangement of the machine suffers from a number of disadvantages, among which are;
(a) complicated percussive action reversal procedure due to simultaneously applying a pull force to the cable for releasing the lock means and rotating the air-feeding hose; two attendants being needed for this operation;
(b) failure to effect a remote control switching from the rearward to the forward percussive action;
(c) impossibility of changing the percussive action mode without terminating the supply of compressed air;
(d) overcomplicated construction;
(e) low operation reliability due to the necessity of an additional means for locking the hammer, which is susceptible to jamming; and
(f) inapplicability in machines of small radial dimensions.
There is further known a percussive action machine (cf., West German Pat. No. 2,105,229; C1. E 02 D 17/146) comprising a cylindrical housing accommodating a reciprocable hammer having a cavity and a port in its tail portion to receive a tube with two rows of holes on its side wall, the interior of the tube being provided with a rotatable rod member having shaped grooves to close or open the ports of the tube. The steps of smaller diameter of the tube sleeve accommodate a pin and a shaped groove. A spring is interposed between the tube and rod member.
In order to reverse the percussive action of the machine (change its operating mode), it is necessary to shut off the delivery of compressed air and reapply the air after a while. Under the action of the spring the rod member is caused to move forward, whereas, the shaped groove of the tube, cooperates with the pin of the rod member to turn the latter. Compressed air admitted to the machine acts to move the rod member rearwards resulting in its turning: after making two revolutions the rod member assumes a new position in which other ports axially offset to the rear of the tube relative to the first group of ports will open (thus closing the first group of ports). This results in a delayed discharge of compressed air from the front working chamber, whereby the machine moves rearwards. For changing the machine operation from the reverse to the forward percussive action the aforedescribed procedures are repeated.
Again, such an arrangement evidence the following disadvantages:
(a) inconveniences in handling because the attendant is not aware of the exact positions assumed by machine parts after stopping; failure to reverse the percussive action of the machine without shutting off the flow of compressed air; each termination of the flow of compressed air to the machine causes a reversal in its percussive action;
(b) insufficient safety when starting to drive the machine into the ground; this is firstly due to the fact that the machine may start with the reverse percussive action to injure the attendant, and secondly a termination in the delivery of compressed air to the machine causes it to reverse, again subjecting the operator to danger;
(c) structural complication necessitated by the fabrication of the shaped groove, such grooves being impossible to make in machines of small diameter; and
(d) low energy of impacts due to the provision of a plurality of ports on the step of larger diameter of the tube requiring an elongated tube and thus reducing the stroke length of the hammer, also overcomplicating the machine structurally.